Huge shale resources available and low gas price turn the oil operators' activities to producing more liquid oil. Common enhanced oil recovery methods can be divided along three different techniques: thermal injection, gas injection, and chemical injection to extract oil from the reservoirs.
Thermal injection uses hot water and steam to extract crude oil from the reservoir. Thermal injection is used for heavily viscous oil that cannot easily flow without adding heat, as the increased temperature reduces the oil's viscosity. Thermal injection has dominated the oil recovery market for 2012 and is utilized heavily by Canada, Indonesia, and California. [TMR 2014]. However, given the high price of the natural gas that is needed to heat the steam, its market share is expected to decrease during the next decade.
Gas injection technology injects gases to extract oil. The most common used gas is carbon dioxide (CO2) since it is an abundant byproduct of industrial processes. In Northern America, many of the carbon dioxide enhanced oil recovery projects are concentrated in West Texas.
Chemical injection technology uses polymer, surfactant solution and alkali to extract crude oil from the reservoirs and can be incorporated in conjunction with another injection method for further efficiency.
Presently, North America leads the World in the enhanced oil recovery market, followed by Europe (especially Russia). Currently, it appears there is no necessity for the Middle East to utilize enhanced oil recovery methods for oil extraction (given the region's abundant resources), this is expected to change and it is anticipated that enhanced oil recovery will play a significant role in the Middle East in the coming years.
It is well known that primary depletion using horizontal wells with multistage fracturing can only produce a few present of the oil in shale reservoirs. Compared to gas flooding, huff-n-puff has more operation parameters to optimize so that liquid oil production can be maximized. Nonetheless, the question as to how to produce the remaining oil has not been addressed or answered. This question becomes more important in the current low oil price, as high-cost drilling, fracturing and completion have to be minimized.
Thus, using existing wellbores to improve remaining oil production becomes more important. Considering different enhanced oil recovery methods, gas injection is probably the most feasible method. [Sheng 2015 A; Sheng 2015 C]. Since Wan 2013 A first proposed cyclic gas injection (huff-n-puff) to improve oil recovery in shale oil reservoirs, many papers have been published on the subject, as reviewed by Sheng 2015 A. Sheng 2014 used a simulation approach to show that cyclic gas injection has the highest potential to enhance oil recovery (EOR) in shale oil reservoirs. Chen 2014 used compositional models of a Bakken formation to simulate CO2 huff and puff. Their results show that the final recovery factor in the huff and puff process is lower than that in the primary recovery, because the incremental recovery in the production stage is unable to compensate the loss in the injection and shut-in stages. In their models, the huff and puff process are from 300 days to 1000 days; the bottom-hole injection pressure is 4000 psi and the producing pressure is 3000 psi. Using this model, Sheng 2015 A was able to repeat Chen 2014 results (the huff-n-puff recovery is lower than the primary recovery). However, this model shows that all the oil recovery factors at the end of 30, 50 and 70 years from the huff and puff process are higher than those from the primary depletion, when the injection pressure of 7000 psi is used. Therefore, Chen 2014's results are caused by the low injection pressure of 4000 psi which is lower than the initial reservoir pressure of 6840 psi. The injection pressure in the high-pressure reservoir should be raised. From this example, a method for enhancement and optimization of huff-n-puff is very important because sometimes a wrong conclusion is made.
Thus, a need remains to improve the huff-n-puff method using a gas (such as methane, natural gas, carbon dioxide, nitrogen, and combinations thereof) to optimize the oil recovery factor in shale reservoirs.